Washington (CNN) -- Congressional Democrats, frustrated that President Obama is including entitlement changes in debt negotiations, pushed back Friday, insisting they will not support any benefit cuts to Medicare or Social Security.

They also reminded the White House it should heed their concerns if it wants to get Democratic votes necessary to pass any agreement.

House Minority Leader Nancy Pelosi, who had a one-on-one meeting with the president Friday morning, told her caucus she reminded Obama that Democrats oppose any reductions in benefits for these government programs that serve as safety nets for the elderly, poor and disabled.

"We are not going to reduce the size of the deficit or subsidize tax cuts for rich, on the backs of America's seniors," Pelosi said at a Capitol Hill news conference.

Instead of flocking to the exits, as is usual on Friday afternoons, many House Democrats remained in the Capitol to attend a closed-door meeting ahead of Sunday's session for bipartisan leaders at the White House. According to several Democratic sources, member after member vented frustration to Pelosi about potentially facing a vote on a $4 trillion package that could include changes to entitlement programs.

After the meeting, the chairman of the Congressional Black Caucus, Emanuel Cleaver, D-Missouri, was unequivocal -- saying he won't vote for any changes to these programs. "We're not going to bend on Medicare, Medicaid, or Social Security."

Seventy House Democrats sent a letter to Obama, asking him to keep Medicare, Medicaid and Social Security off the bargaining table.

Rep. Jim McGovern, D-Massachusetts, said he was caught off guard when he learned the White House was proposing major entitlement reforms as part of a debt limit deal. "All of the sudden, I read this in the paper two days ago that this in on the table and someone is going to come up with a reform in a weekend?" he said.

"I'm a Democrat. I came to Congress to protect Social Security and Medicare, not to see it dismantled," McGovern added.

Virginia Democratic Rep. Gerry Connolly described his colleagues as "understandably very skittish about putting Medicare back on the table and Social Security while they're at it, which has very little to do with the debt."

Some Democrats focused their concerns on reports that the White House was considering changing the way Social Security benefits are calculated.

"The notion that the way you're going to balance the budget is to make it more difficult for senior citizens to get cost of living adjustment increases, what you're saying is we're going to lower the standard of living for seniors in this country," McGovern said.

But Pelosi stressed that any potential changes to Social Security are "hypothetical," telling reporters, "The dirty rotten devil is in the dirty rotten details."

While Democrats continued to draw a line at changing benefits, they did leave room for other changes to Medicare, such as savings from overpayments and waste in the program, revising rules for the way the program buys prescription drugs, and changes for those people who receive benefits under both Medicare and Medicaid.

But Pelosi insisted that any savings accrued had to be reinvested back into Medicare and Social Security, not directed to tax cuts or deficit reduction.

Connolly acknowledged that signing onto a bipartisan agreement that included Medicare changes could also cost Democrats politically. That's because Democrats have been railing against changes to the program proposed by Republicans -- and believe that voter concerns about Medicare could help them win in the next election.

Rep. Chris Van Hollen, D-Maryland, seemed less concerned about losing the advantage on the issue, saying Democrats would never agree to anything as far reaching as the Republican plan. "Whatever agreement is reached, it's not going to include anything like the House Republican plan that guts Medicare and passes on these huge costs," he said

Several Democrats predicted that Obama would need at least 100 House Democrats to vote for the deal to get it through the House, a task Connolly called "a tall order."

Rep. Rob Andrews, D-New Jersey, said getting that many Democrats to vote 'yes' would be "very hard." He said the president "needs to explain the stakes of not acting."

While Democrats made it clear their votes should not be taken for granted, House Speaker John Boehner of Ohio also faces a steep climb convincing House Republicans to vote for any revenue increases.

The head of the House GOP's campaign committee, Pete Sessions of Texas, underlined the challenge, saying the vote on the debt ceiling "will be the biggest issue that will face members of Congress in their re-election."

One House GOP leadership aide said both House Majority Leader Eric Cantor and the No. 2 Senate Republican, John Kyl, expressed reservations at Thursday's White House meeting about the prospects for getting a $4 trillion package through Congress if it includes a trillion in tax revenues.

Such a high figure, the aide noted, would mean Republicans would have to agree to proposals that go much farther than simply closing loopholes and would go against a GOP pledge not to raise taxes.